Integrity verification is well known in the computer/cryptographic fields. Typical applications are to ensure that a message that is transmitted is received without being corrupted accidentally (due to errors caused by the communications channel) or maliciously such as by a hacker. The same verification techniques are used to make sure that the message (or other data including software programs) that has been stored, such as in computer memory, has not been similarly corrupted. Hackers corrupt computer data or software for two reasons: one is mere vandalism, to render the corrupted material worthless to its owner. The other is more subtle and important, in that hackers tend to tamper with the software programs (‘code’) of others by planting therein their own bits of code that allow the hackers to illicitly manipulate or use for their own purposes the tampered-with software.
An example of such tampering is that distributors of music and video content using the Internet or other computer networks do so using a Digital Rights Management system (DRM) to protect the content from illicit copying and use. DRM is used to protect digital content transferred over a network and transferred from a computer to an associated playback device. The DRM is implemented by software resident in the audio/video player or associated computer, and hence is available for tampering by a hacker, who thereby can “crack” the DRM and render it useless. For instance, the DRM tends to use cryptographic techniques to restrict usage to those who have been granted a right to the content, typically pieces of music or video programs, and cracking it may make the deciphering keys available to the hacker for unauthorized use of the content.
For many digital file transfer situations, a hacker who legitimately receives the content may try thereby to break the DRM protection scheme, so as to give illicit access to third parties. An identified weak link in DRM security is the overall process, rather than the encryption scheme itself. Thus hackers may try to corrupt the DRM software to gain illicit access to the content so it is for instance stripped of the encryption or other protective mechanisms, using the above described tampering approach.
Such corruption of computer/data files is a well known problem with a well known solution, known as integrity verification. This involves taking the file as a whole, or a fixed length part thereof, and computing a checksum or hash value for that part. A checksum is a form of redundancy check, and is a simple measure to protecting the integrity of data by allowing one to later detect errors in data sent through space (a communication) or time (stored and later retrieved.) It operates by adding up the values of the basic components of the file or part thereof (which are already in numerical, typically binary, form) and storing the resulting total value. Later, anyone can perform the same checksum operation on the data, compare the result to the authentic checksum, and if the totals match, conclude that the data was likely not corrupted. Simple checksums cannot detect many types of errors, such as reordering of portions of the data, inserting or deleting zero values, or multiple errors which sum to zero.
More sophisticated types of redundancy checks are known to overcome these weaknesses. Even they are not of much use against hackers, since their simple and known operations are easy to circumvent. To provide a higher level of integrity against hackers, the use of a cryptographic hash function (one way function) known as an HMAC (hash message authentication code) is well known in the field. Examples are MD5 (Message Digest version 5) and Secure Hash Algorithm 1 (SHA 1) generally referred to hereinafter as hash or hash functions. These are generally secure in a mathematical sense against hackers due to the complexity of the calculation of the hash value, which is used instead of the simpler checksum. However, like most advanced cryptographic techniques, such hash functions are computationally intensive thus being slow or requiring substantial computer performance, and while suitable for transmission and protection of high value material, may not be suitable for the commercial distribution of audio/video material which must be played on a compact and inexpensive dedicated player.